Adenoviruses (Ads) are common human viruses that cause mainly respiratory infections, as well as gastrointestinal infections and severe ocular infections. Modified forms of adenovirus have shown great potential for gene delivery and vector-based vaccination strategies. Ads provide numerous advantages for vector design including a broad host range, the ability to infect both replicative and non-replicative cells, and the fact that they do not integrate into the host chromosome so they do not inactivate genes or activate oncogenes. Understanding the underlying cell entry mechanism and tissue specificity of Ad is critical for engineering improved vectors. A key aspect of this proposal is its collaborative approach with an established adenovirologist to correlate structural findings with functional analyses. Cryo-electron microscopy (cryoEM) offers a powerful way to visualize Ad and Ad complexes with host factors including key antimicrobial peptides. The specific aims of this proposal are to 1) determine a subnanometer (6-7[unreadable]) resolution cryoEM structure of the complex of HD5 with a chimeric species B/C Ad vector, Ad35F, and 2) determine a subnanometer (6-7[unreadable]) resolution cryoEM structure of the complex of HD5 with a chimeric species C/D Ad vector. Defensins are immune peptides that present the first line of defense against invading microbes. Understanding the molecular principles of this natural defense mechanism could lead to novel strategies for antiviral drug design.